268 THE FERRIC-OXALATE PHOTOMETER. [MEMOIR XVIII. 



but also as respects physical geography and the great 

 interests of agriculture. The sum of vegetable organi- 

 zation is in all climates and localities a function of the 

 light distributed thereto. And so far as heat is con- 

 cerned, it is not the intensity only, but the absolute 

 quantity, which is to be measured. To each plant, from, 

 the moment of its germination to the moment of its 

 maximum development and the completion of its life, a 

 definite quantity of heat and of light must be given. As 

 respects the heat in such inquiries, it is not only the 

 thermometer but the calorimeter which must be consid- 

 ered; and as to the light, the photometers herein de- 

 scribed determine its quantity but not its brilliancy, 

 and therefore answer the indications required. And 

 since it is not merely the temperature of a locality, but 

 also the light, of the sun, which is the effective condition 

 of vegetable growth, we see how important even in agri- 

 culture itself these proposed determinations really are. 



To those who would devote themselves to such inqui- 

 ries I recommend as a photometric means a mixture of 

 chlorine and hydrogen where great sensitiveness is re- 

 quired, and in other cases ferric oxalate. 



The chlor-hydrogen and the ferric oxalate photometers 

 act by selective absorption, on the principle of the calo- 

 rimeter; that is to say, they measure the quantity of the 

 radiations they select. 



I may, perhaps not inappropriately, close this Memoir 

 with a brief allusion to an instrument I formerly used 

 very much. It measures the intensity of the radiations 

 it is made to select, and these radiations may be varied 

 at pleasure. I will describe it first as adjusted for the ra- 

 diations of which chlor-hydrogeu and ferric oxalate take 

 charge. The description is extracted from a paper I pub- 

 lished in the Philosophical Magazine, August, 1844. 



